1. Field of the Invention
The present invention relates to a method for producing a preform for a polarization retaining optical fiber which is used in an optical fiber laser or an optical fiber light amplifier.
2. Description of the Related Art
As a method for producing a preform for a polarization retaining optical fiber, Japanese Patent Kokai Publication No. 155535/1985 discloses a method comprising boring at least two bores which are symmetrically arranged around a center axis of a preform for use in the fabrication of an optical fiber, smoothing inner surfaces of the bores, and inserting and fixing stressing members having a different coefficient of thermal expansion from that of a core and cladding parts of the preform in the bores. The stressing member is fixed in the bore by making an outer diameter of one end of the bore larger than an inner diameter of the bore and inserting and fixing said one end of the stressing member to an end opening of the bore.
In general, since a material of the stressing member has a very much different coefficient of thermal expansion from that of the material of the preform for the optical fiber, the stressing member deforms much larger than the preform material when they are heated to integrate them together.
In the above conventional fixing manner, since the whole stressing member is not completely fixed to the preform though the one end of the stressing member is inserted in the bore, an unfixed part of the stressing member deforms largely, so that the stressing member moves during the integration of the stressing member and the preform and drawing of the preform having the stressing members. As the result, a desired stress is not achieved in the drawn fiber and no good polarization retaining property is realized because of the decrease of the diameter of the stressing member or deformation of the stressing member.